This invention relates to polycrystalline metal oxide varistors. More particularly, this invention relates to a novel configuration of polycrystalline metal oxide varistors in which the voltage overshoot, which occurs when the device is subjected to voltage pulses of extremely rapid rise time, is reduced.
The polycrystalline metal oxide varistor is a device which has a very non-linear voltage-current characteristic which makes it a very useful device for fixedly clamping the voltage across it and the device which it is to protect at a predetermined, safe value. That is to say, for voltage values below this clamping voltage, the device behaves like an ohmic resistor of very large value (as much as approximately 10,000 megohms) but when this clamping voltage is exceeded the device behaves like a low resistance conductor. The current voltage relationship which exists in these devices is expressed by the following equation: EQU I = (V/C).sup..alpha.
where
I is the current flowing through the material,
V is the voltage across the material,
C is a constant which is a function of the physical dimensions of the body of the device, its composition, and the parameters of the process employed to form the body, and is a measure of the clamping voltage, and
.alpha. IS A CONSTANT FOR A GIVEN RANGE OF CURRENT AND IS A MEASURE OF THE NON-LINEARITY OF THE RESISTANCE CHARACTERISTIC OF THE BODY.
Metal oxide varistors are sintered ceramics composed principally of zinc oxide with a mixture of various other metal oxides added. These other oxides are typically bismuth trioxide, cobalt trioxide, manganese dioxide, antimony trioxide, and tin dioxide, each being present to the extent of approximately one-half to one mol percent, the remainder of the material being zinc oxide. The powder is ground and pressed into the desired shape after which the material is sintered at a temperature of approximately 1,000.degree. C. to 1,400.degree. C. After this, electrodes are applied to faces of the material and circuit connection wires are attached to the surface electrodes. The materials and processes for making such devices are well known in the art and are described, for example, in U.S. Pat. No. 3,962,144, issued to Matsurra et al.
The metal oxide varistor, in many respects, is similar to a bidirectional Zener diode, in that when a voltage is applied in excess of a certain value, the characteristic resistance of the device decreases dramatically and conduction through the device occurs. This nonlinear conduction characteristic renders the metal oxide varistor an important element in a variety of protective circuit configurations.
One of the most important charateristics of the metal oxide varistor is the voltage at which clamping occurs, the so-called breakdown voltage of the device. It is this value that determines the degree of protection the device offers. The voltage pulses to which the varistor is subjected are characterized by their rise times. The rise time of such a pulse is defined here and in the electrical arts as the time it takes the waveform to rise from 10 percent to 90 percent of its peak value. Thus, it is also known that when voltage pulses having extremely short rise times (approximately 0.1 .mu.sec. or less) are applied across the device, that there is a period during which the voltage is clamped not at the steady state clamping voltage value but rather at a voltage which can be as much as 30 percent in excess of this value. However, for rise times found in most varistor applications, that is, approximately 0.1 .mu.sec., the voltage overshoot is typically in the range from 3 percent to 10 percent. For example, a varistor with a nominal clamping voltage of 600 volts, when subjected to a voltage pulse whose rise time is approximately 0.1 .mu.sec., with a 30 percent overshoot, develops a voltage across the varistor of 780 volts which excess voltage persists for approximately 1 .mu.sec. Hence, this overshoot is a highly undesirable characteristic, if for adequate circuit protection, the voltage across the protected device cannot exceed 600 volts for such a long duration.
Another important characteristic of metal oxide varistors is their upturn. That is to say, they do not exactly obey the current-voltage relationship expressed in the equation above, but rather at high levels of current the value of the exponent alpha becomes lower. This means that the voltage across the device increases faster than it otherwise would for higher values of current.
Another important characteristic of the metal oxide varistors is their leakage current. When a varistor is operating below its clamping or breakdown voltage, it should exhibit a high resistance characteristic which means that for a given voltage in this range, the current through the device should be minimal. Typically, the current that flows through the varistor at one-half of its clamping voltage is referred to as the leakage current.
It is well known in the art of varistor manufacture, that the clamping voltage of the varistor is determined by the number of intergranular boundaries between crystals of zinc oxide in the varistor substrate. These crystals range in size from between 5 and 50 microns, the typical grain size being approximately 25 microns. It is known that each such grain boundary contributes a clamping voltage of approximately 3 volts. Hence, a 5 millimeter slab of a sintered ceramic oxide varistor material possesses approximately 200 such grain boundaries on the average, assuming a grain size of 25 microns. This results in a clamping voltage of approximately 600 volts.
However, the overshoot problem still persists and if a sufficiently high and sufficiently fast voltage pulse occurs, an overshoot as large as 30 percent permits the voltage to rise up to a level of 780 volts, depending on composition and the rate of rise, with more typical values of overshoot being in the range of from 3 percent to 10 percent.